On occasion, hydraulic hoses connected to hydraulic cylinders in a device such as a scissors lifts fail. Failure of a hydraulic input hose results in a sudden pressure loss wherein hydraulic fluid can rapidly flow from the hydraulic cylinder allowing the scissors lift collapse. This can, of course, result in injury to personnel and in equipment damage.
In order to minimize the chance of a hydraulically powered, lift device collapsing, hydraulic fuses are utilized which prevent back flow through hydraulic lines upon a loss of pressure. In the case of a lift, such as a scissors lift, the hydraulic fuse is moved to a blocking position by the weight of the raised lift which pushes the piston in the hydraulic cylinder used to accomplish the lift back into the cylinder. This ejects fluid from the cylinder moving the fuse to block flow of hydraulic fluid out of the cylinder.
Once the hydraulic system is blocked, the hydraulic lift cannot collapse. This leaves the operator suspended above the ground. In order to lower the operator, the system is pumped. This requires that a separate valve be connected to the hydraulic line.